This invention relates to d.c. motors and more particularly to a temperature control circuit for a d.c. motor.
D.C. motors, for example those used on cranes or as traction motors on vehicles such as fork lift trucks, are often, in some conditions of operation, run at power levels which are greater than their normal design limits. In such conditions, the high motor current can cause the motor to overheat, with the risk of damage to the motor. In the past, this problem has usually been overcome by continuously monitoring the temperature of the motor by means of a temperature sensing element, such as a thermistor or a thermally operated switch, positioned within the motor frame, the sensing element supplying a signal to the motor control circuits when overheating occurs. This has the disadvantage that the sensing element must be embodied within the motor during manufacture and that additional motor terminals are required through which the information may be extracted from the element. In addition, it is difficult to replace the sensing element if it develops a fault in use. It is desirable to sense overheating of a d.c. motor without the need to embed a sensing element within the motor.